I'm currently studying a chapter on Differentiation. I was introduced to first prinicples, and i found a few of the first questions easy. But, ive hit a dead end, and nothing I try with this next question works. I looked up the method and found explanations at:

First Principles

However, this does not help me get the answer I seek. Could you please help?

The question goes:

"Use First principles to find

Now i used the formula, and got something like this:

From here, I have no idea, Ive tried many different ways and I always end up with the wrong answer.

Please explain to me how to do this.